Low-voltage differential amplifier

ABSTRACT

A low-voltage differential amplifier circuit is disclosed. The low-voltage differential amplifier circuit includes a first differential amplifier, a second differential amplifier and a summing circuit. The first differential amplifier receives a pair of differential input signals to generate a first output. The second differential amplifier receives the same pair of differential input signals to generate a second output. The summing circuit sums the first output of the first differential amplifier and the second output of the second differential amplifier to provide a common output.

BACKGROUND OF INVENTION

1. Technical Field

The present invention relates to electronic circuits in general, and, inparticular, to differential amplifier circuits. Still more particularly,the present invention relates to a low-voltage differential amplifiercircuit having a wide common mode range.

2. Description of Related Art

A conventional differential amplifier circuit typically includes a pairof field effect transistors with the gates as differential inputs andthe drains as outputs. The conventional differential amplifier circuitalso includes a current source connected to the sources of the fieldeffect transistors to supply a fixed current to the field effecttransistors. During operation, the differential voltage inputs suppliedto the gates are compared, and the conductivity of one of the fieldeffect transistors is raised whilst the conductivity of the othertransistor is lowered.

Conventional differential amplifier circuits are best operated in acommon mode range (CMR) centered around V_(dd)/2. When outside theabove-mentioned CMR, the outputs of conventional differential amplifiercircuits may not be acceptable. Such limitation in the CMR tend torestrict the type of applications conventional differential amplifiercircuits can be employed.

There are several solutions for increasing the CMR of a conventionaldifferential amplifier circuit. However, none of the solutions allowproper operations from 0.1 to (V_(dd)−0.2) volt over the entire process,voltage and temperature (PVT) operating range. Consequently, it would bedesirable to provide an improved differential amplifier circuit having arelatively wide common mode range.

SUMMARY OF INVENTION

In accordance with a preferred embodiment of the present invention, adifferential amplifier circuit includes a first differential amplifier,a second differential amplifier and a summing circuit. The firstdifferential amplifier receives a pair of differential input signals togenerate a first output. The second differential amplifier receives thesame pair of differential input signals to generate a second output. Thesumming circuit sums the first output of the first differentialamplifier and the second output of the second differential amplifier toprovide a common output for the differential amplifier circuit.

All objects, features, and advantages of the present invention willbecome apparent in the following detailed written description.

BRIEF DESCRIPTION OF DRAWINGS

The invention itself, as well as a preferred mode of use, furtherobjects, and advantages thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment whenread in conjunction with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a differential amplifier circuit, inaccordance with a preferred embodiment of the present invention;

FIG. 2 is a circuit diagram of a reference voltage generation circuit tobe used with a summing circuit within the differential amplifier circuitfrom FIG. 1, in accordance with a preferred embodiment of the presentinvention; and

FIG. 3 graphically compares the common mode range of the differentialamplifier circuit from FIG. 1 with the common mode range of aconventional differential amplifier circuit.

DETAILED DESCRIPTION

Referring now to the drawings and in particular to FIG. 1, there isdepicted a circuit diagram of a differential amplifier circuit, inaccordance with a preferred embodiment of the present invention. Asshown, a differential amplifier circuit 10 includes a first differentialamplifier 11, a second differential amplifier 21 and a summing circuit31. First differential amplifier 11 and second differential amplifier 21are connected in parallel with each other and both share the respectivedifferential inputs. The outputs of first differential amplifier 11 andsecond differential amplifier 21 are combined by summing circuit 31 toproduce a single common output for differential amplifier circuit 10.

First differential amplifier 11 includes p-channel transistors 12-14 andn-channel transistors 15-17. P-channel transistor 13 is connected inseries with n-channel transistor 15. Similarly, p-channel transistor 14is connected in series with n-channel transistor 16. The gate and drainof p-channel transistor 13 are connected to the gate of p-channeltransistor 14. P-channel transistor 12 is connected between Vdd and thedrains of p-channel transistors 13-14. N-channel transistor 17 isconnected between the sources of n-channel transistors 15-16 and ground.The gate of n-channel transistor 17 is connected to V_(CMN)-a gatecontrol voltage to control the current through n-channel transistor 17in a consistent and predictable manner using a current mirror technique.The gate of p-channel transistor 12 is connected to an active lowENABLE_N signal. Because n-channel differential amplifiers 15-16 receivedifferential input pair-input+ and input−, first differential amplifier11 is considered as an n-channel differential amplifier.

Second differential amplifier 21 includes p-channel transistors 22,25-27 and n-channel transistors 23-24. P-channel transistor 25 isconnected in series with n-channel transistor 23. Similarly, p-channeltransistor 26 is connected in series with n-channel transistor 24. Thegate and drain of n-channel transistor 23 are connected to the gate ofn-channel transistor 24. P-channel transistors 22 and 27 are connectedin series between V_(dd) and the sources of p-channel transistors 25-26.The gate of n-channel transistor 27 is connected to V_(CMP)—a gatecontrol voltage to control the current through p-channel transistor 27in a consistent and predictable manner using a current mirror technique.The sources of n-channel transistors 23-24 are connected to ground. Thegate of p-channel transistor 22 is connected to ENABLE_N signal. Becausep-channel differential amplifiers 25-26 receive differential inputpair-input+ and input−, second differential amplifier 21 is consideredas a p-channel differential amplifier.

Summing circuit 31 includes p-channel transistors 32-34 and n-channeltransistors 35-38. P-channel transistor 33 is connected in series withn-channel transistor 35. Similarly, p-channel transistor 34 is connectedin series with n-channel transistor 36. The gate and drain of p-channeltransistor 33 are connected to the gate of p-channel transistor 34.N-channel transistors 37-38 are connected in series between the sourcesof n-channel transistors 35-36 and ground. The gate of n-channeltransistor 37 is connected V CMN. The gate of n-channel transistor 35 isconnected to a reference voltage V_(ref). Reference voltage V_(ref)provides proper voltage biasing for summing circuit 31. Referencevoltage V_(ref) is preferably generated by a reference voltagegeneration circuit shown in FIG. 2. The gate of n-channel transistor 36is connected to the output of first differential amplifier 11 (i.e., thenode between p-channel transistor 14 and n-channel transistor 16) and tothe output of second differential amplifier 21 (i.e., the node betweenp-channel transistor 26 and n-channel transistor 24). The drain ofp-channel transistor 32 is connected to the node between p-channeltransistor 34 and n-channel transistor 36 to provide a single commonoutput 39 for differential amplifier circuit 10. The gates of p-channeltransistor 32 and n-channel transistor 38 are connected to an active lowENABLE_P signal. P-channel transistor 32 is a clamp device to holdoutput 39 high when ENABLE_P is low (i.e., inactive).

During operation, an active low ENABLE_N signal is applied to the gatesof p-channel transistors 12 and 22, and an active high ENABLE_P signalis applied to p-channel transistor 32 and n-channel transistor 38. Apair of differential input signals are applied to the gates of n-channeltransistor 15 and p-channel transistor 25 via input+, and to the gatesof n-channel transistor 16 and p-channel transistor 26 via inputB,respectively. The output for first differential amplifier 11 isgenerated at the node located between p-channel transistor 14 andn-channel transistor 16. Concurrently, the output for seconddifferential amplifier 21 is generated at the node located betweenp-channel transistor 26 and n-channel transistor 24. The outputs fromfirst differential amplifier 11 and second differential amplifier 21,which are analog in nature, are subsequently combined by summing circuit31 at the gate of n-channel transistor 36. In turn, summing circuit 31provides a common output signal at output 39 for differential amplifiercircuit 10.

With reference now to FIG. 2, there is depicted a circuit diagram of areference voltage generation circuit for summing circuit 31, inaccordance with a preferred embodiment of the present invention. Asshown, a reference voltage generation circuit 20 includes p-channeltransistors 41-44 and n-channel transistors 45-46. P-channel transistors41-42 are connected in series between V_(dd) and the sources ofp-channel transistors 43-44. The gate of p-channel transistor 41 isconnected to active low ENABLE_P signal. The gate of p-channeltransistor 42 is connected to V_(CMP). P-channel transistor 43 isconnected in series with n-channel transistor 45. Similarly, p-channeltransistor 44 is connected in series with n-channel transistor 46. Thegate and drain of n-channel transistor 45 are connected to the gate ofn-channel transistor 46. The sources of n-channel transistors 45-46 areconnected to ground. The gate and drain of p-channel transistor 44 areconnected to the gate of p-channel transistor 43 to generate a referencevoltage V_(ref) for summing circuit 31 (from FIG. 1).

Referring now to FIG. 3, there is illustrated a graphical comparisonbetween the common mode range of differential amplifier circuit 10 (fromFIG. 1) and the common mode range of a conventional differentialamplifier circuit. As shown, a pair of differential±50 mV input signals51 is sent to both a conventional differential amplifier circuit anddifferential amplifier circuit 10, ramping from 0.0 V to approximately0.8 V. The common mode range of the conventional differential amplifiercircuit, as indicated by an output waveform 52, is not acceptable withinthe input voltage range of 0.1 V to 0.4 V. In contrast, the common moderange of differential amplifier circuit 10, as indicated by an outputwaveform 53, remains consistent throughout the input voltage range ofapproximately 50 mV through 0.8 V.

As has been described, the present invention proa low-voltagedifferential amplifier circuit having a relatively wide common moderange. The differential amplifier circuit of the present inventionallows a switch point reference to be established by shorting twodifferential inputs to a single output, causing the entire differentialamplifier to hang-up at the unity gain switch point. The output is thenutilized as the reference voltage V_(ref) input of the summing circuitto provide a nearly perfect centered reference point that tracks wellover the entire voltage, process, temperature and common mode operatingrange.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

1. A differential amplifier circuit comprising: a first differentialamplifier for receiving a pair of differential input signals to generatea first output; a second differential amplifier for receiving said pairof differential input signals to generate a second output; a summingcircuit for summing said first output of said first differentialamplifier and said second output of said second differential amplifierto provide a common output for said differential amplifier circuit; anda reference voltage generation circuit for providing a reference voltagesignal to said summing circuit, wherein said reference voltagegeneration circuit is a differential amplifier.
 2. The differentialamplifier circuit of claim 1, wherein said first differential amplifieris an n-channel differential amplifier.
 3. The differential amplifiercircuit of claim 2, wherein said first differential amplifier includes apair of n-channel transistors for receiving said pair of differentialinput signals, respectively.
 4. The differential amplifier circuit ofclaim 1, wherein said second differential amplifier is a p-channeldifferential amplifier.
 5. The differential amplifier circuit of claim1, wherein said second differential amplifier includes a pair ofp-channel transistors for receiving said pair of differential inputsignals, respectively.
 6. The differential amplifier circuit of claim 1,wherein said summing circuit is an n-channel differential amplifier. 7.The differential amplifier circuit of claim 1, wherein said summingcircuit includes an n-channel transistor pair, wherein a firsttransistor of said n-channel transistor pair receives said voltagereference signal from said reference voltage generation circuit, whereina second transistor of said n-channel transistor pair receives combinedoutput signals from said first output of said first differentialamplifier and said second output of said second differential amplifier.8. The differential amplifier circuit of claim 1, wherein said referencevoltage generation circuit is a p-channel differential amplifier.
 9. Thedifferential amplifier circuit of claim 8, wherein said referencevoltage generation circuit receives an active low ENABLE_P signal. 10.The differential amplifier circuit of claim 1, wherein said first andsecond differential amplifiers receive an active low ENABLE_N signal.11. The differential amplifier circuit of claim 10, wherein said summingcircuit receives an active low ENABLE_P signal.
 12. The differentialamplifier circuit of claim 11, wherein said summing circuit includes aclamp device to hold said common output high when said ENABLE_P signalis low.
 13. The differential amplifier circuit of claim 1, wherein saidfirst differential amplifier receives a gate control voltage V_(CMN) tocontrol the current through an n-channel transistor within said firstdifferential amplifier in a consistent and predictable manner using acurrent mirror technique.
 14. The differential amplifier circuit ofclaim 1, wherein said second differential amplifier receives a gatecontrol voltage V_(CMP) to control the current through a p-channeltransistor within said second differential amplifier in a consistent andpredictable manner using a current mirror technique.
 15. Thedifferential amplifier circuit of claim 1, wherein said summing circuitreceives a gate control voltage V_(CMN) to control the current throughan n-channel transistor within said summing circuit in a consistent andpredictable manner using a current mirror technique.